Refrigeration Cycle, Components, and Refrigerants
Follow the refrigerant
The cycle has four functions: compression (low-pressure vapor to high-pressure vapor), condensation (rejecting heat, vapor to liquid), metering (pressure drop), and evaporation (absorbing heat, liquid to vapor). Trace it on a pressure-enthalpy diagram and know the states: desuperheated vapor leaving the compressor, subcooled liquid leaving the condenser, a flash-gas mixture after the metering device, and superheated vapor leaving the evaporator.
The two measurements that diagnose everything
Superheat is measured at the evaporator outlet: actual line temperature minus the saturation temperature for the suction pressure. Subcooling is measured at the condenser outlet: saturation temperature for the liquid-line pressure minus actual liquid-line temperature.
This is the highest-yield trap in the domain: on a fixed-orifice or cap-tube system, charge is verified by superheat; on a TXV/EEV system, charge is verified by subcooling, because the valve holds superheat roughly constant. Typical targets are 8–12°F superheat at the evaporator and 10–15°F subcooling at the condenser, but always use the manufacturer's charging chart.
Refrigerants and handling
R-410A is a near-azeotropic blend charged as a liquid; R-22 is a single-component HCFC no longer produced for new equipment; R-134a serves medium-temperature work; A2L refrigerants such as R-32 and R-454B are mildly flammable and require ignition-source awareness. Always recover, never vent. For small appliances under 5 lb, recovery must reach 80% of the charge or 4 in. Hg vacuum. Evacuate to 500 microns and perform a standing vacuum test; pressure-test with dry nitrogen, never with refrigerant alone.
Worked example: liquid-line pressure corresponds to 110°F saturation and the liquid line measures 95°F, so subcooling is 110 − 95 = 15°F. On a TXV system that reads high - suspect an overcharge or a restricted condenser.